The present invention relates to a technical field of stimulable phosphor panels and, more specifically, a stimulable phosphor panel with a transparent moisture-resistant protective layer for sealing a stimulable phosphor layer therewith.
There are known a class of phosphors which accumulate a portion of applied radiations (e.g. x-rays, α-rays, β-rays, γ-rays, electron beams, and uv (ultraviolet) radiation) and which, upon stimulation by exciting light such as visible light, give off a burst of light emission in proportion to the accumulated energy. Such phosphors called stimulable phosphors are employed in medical and various other applications.
An exemplary application is a radiographic image information recording and reproducing system which employs a stimulable phosphor panel having a film formed of the stimulable phosphor (stimulable phosphor layer, hereinafter referred to simply as phosphor layer). The stimulable phosphor panel is hereinafter referred to as the phosphor panel and also called the radiographic image conversion sheet. This radiographic image information recording and reproducing system has already been commercialized as FCR (Fuji Computed Radiography) from Fuji Photo Film Co., Ltd.
In that system, a subject such as a human body is irradiated with x-rays or the like to record radiographic image information about the subject on the phosphor panel (more specifically, the phosphor layer). After the radiographic image information is thus recorded, the phosphor panel is scanned two-dimensionally with exciting light such as laser light to produce stimulated emission which, in turn, is read photoelectrically to yield an image signal. Then, an image reproduced on the basis of the read image signal is output as the radiographic image of the subject, typically to a display device such as CRT or on a recording material such as a photographic material.
The phosphor panel is typically produced by the steps of first preparing a coating solution having the particles of a stimulable phosphor dispersed in a solvent containing a binder, etc., applying the coating solution to a support in panel form that is made of glass or resin, and drying the applied coating.
Phosphor panels are also known that are made by forming a phosphor layer on a support through methods of vacuum film deposition (vapor-phase film formation) such as vacuum evaporation or sputtering (see JP 2789194 B and JP 5-249299 A). The phosphor layer prepared by the vacuum film deposition has excellent characteristics. First, it contains less impurities since it is formed under vacuum; further, it is substantially free of any substances other than the stimulable phosphor, as exemplified by the binder, so it has high uniformity in performance and still assures very high luminous efficiency.
One factor for deterioration of characteristics of the phosphor panel is moisture absorption by the stimulable phosphor layer. The stimulable phosphor layer, in particular, the alkali halide-based stimulable phosphor layer having favorable characteristics, has high moisture absorption and easily absorbs moisture even in a normal environment (normal temperature/normal humidity). As a result, deterioration of sharpness of a reproduced image or the like occurs due to deterioration of photostimulated luminescence characteristics, that is, sensitivity, or deterioration of crystallinity of the stimulable phosphor (destruction of columnar crystals in the case of the alkali halide-based stimulable phosphor having a columnar structure, for example).
Some of known stimulable phosphor panels avoid such inconveniences by using a transparent moisture-resistant sheet as a protective layer and sealing a stimulable phosphor layer in an airtight area between the transparent moisture-resistant protective layer and a substrate.
However, when used in a high-altitude place where the atmospheric pressure is low or in a high-temperature environment, a stimulable phosphor panel with its stimulable phosphor layer hermetically sealed between a transparent moisture-resistant protective layer and a substrate causes gas in the airtight area between the transparent moisture-resistant protective layer and the substrate to expand to enlarge the distance between the respective layers, which may warp the stimulable phosphor layer.
For instance, when there is a gap between a transparent moisture-resistant protective layer and a stimulable phosphor layer in a stimulable phosphor panel, excitation light with which the stimulable phosphor layer is irradiated through the transparent moisture-resistant protective layer in order to read a radiographic image recorded on the stimulable phosphor layer is repeatedly reflected between the stimulable phosphor layer and the transparent moisture-resistant protective layer. Therefore, the area of the stimulable phosphor layer which is irradiated with excitation light varies with the length of the gap between the stimulable phosphor layer and the transparent moisture-resistant protective layer which is changed by the expansion of gas or the like.
FIG. 3 shows how excitation light Le is reflected when a gap 120 between a transparent moisture-resistant protective layer 118 and a stimulable phosphor layer 114 is wide. In order to read a radiographic image recorded on the stimulable phosphor layer 114, the excitation light Le reflected between the transparent moisture-resistant protective layer 118 and the stimulable phosphor layer 114 spreads-over an area R1.
On the other hand, FIG. 4 shows how the excitation light Le is reflected when the transparent moisture-resistant protective layer 118 and the stimulable phosphor layer 114 are close to each other. In this case, the distance the excitation light travels each time it is reflected is short. Accordingly, the excitation light Le is reflected between the transparent moisture-resistant protective layer 118 and the stimulable phosphor layer 114 within an area R2, which is smaller than the area R1.
Thus, even if excitation light is reflected the same number of times between the transparent moisture-resistant protective layer 118 and the stimulable phosphor layer 114, the area irradiated with the excitation light varies with the distance between the two layers. More specifically, the area irradiated with excitation light changes with the distance between the transparent moisture-resistant protective layer 118 and the stimulable phosphor layer 114.
For example, when the gap between the transparent moisture-resistant protective layer 118 and the stimulable phosphor layer 114 increases, the stimulable phosphor layer 114 generates as much photostimulated luminescence as when irradiated with a flux of excitation light thick enough to cover the wide area R1, which is larger than a photostimulated luminescence detection area where photostimulated luminescence is detected to obtain image signals and form a radiographic image from the image signals. The photostimulated luminescence outside of the detection area is also detected and lowers the sharpness of the resultant radiographic image.
The stimulable phosphor layer 114 may often be warped (lifted) due to expansion of gas in the airtight area if the stimulable phosphor layer 114 is bonded to the transparent moisture-resistant protective layer 118. This state is conceptually shown in FIG. 5. The stimulable phosphor layer 114 is irradiated with excitation light at a given point H1 and, once warped, at a point H2 which is different from the point H1.
Once warped, the stimulable phosphor layer 114 that is to be irradiated with the excitation light Le at the given point H1 (a point H1′ on the warped stimulable phosphor layer 114) is irradiated at a different point from the point H1, that is, the point H2, and photostimulated luminescence from the point H2 is detected by a detection means 122 which is set so as to detect photostimulated luminescence from the point H1.
As a result, the detection means 122 has lowered efficiency in collecting photostimulated luminescence. Furthermore, image signals obtained at the point H2 from which the photostimulated luminescence is emitted and which is offset from the given point H1′ on the warped stimulable phosphor layer 114 (H1 prior to the warping), form a distorted radiographic image, making it difficult to read a radiographic image recorded on the stimulable phosphor layer 114 accurately.
In order to solve this problem, the inventor of the present invention has provided a stimulable phosphor panel capable of avoiding moisture absorption in a stimulable phosphor layer and stopping changes in temperature and atmospheric pressure from lowering the quality of image signals read from the stimulable phosphor layer (see US 2003/0160188 A). This stimulable phosphor panel has a buffer space that can freely expand or contract and that communicates with an airtight area where the stimulable phosphor layer is formed between a substrate and a transparent moisture-resistant protective layer.
A specific example of this panel is shown in FIG. 6. A stimulable phosphor panel 140 of FIG. 6 has a substrate 112 with a ventilation hole 126 formed so as to communicate with an airtight area where a stimulable phosphor layer 114 is formed. A moisture-resistant cylinder 144 is connected to the ventilation hole 126 and fixed to the rear side of the substrate 112. Inserted in the cylinder 144 is a piston 142 which seals the cylinder 144 and can move freely along the length of the cylinder 144. A closed space formed inside the cylinder 144 by the piston 142 and the cylinder 144 is a buffer space 148.
Another example is shown in FIG. 7. A stimulable phosphor panel 150 of FIG. 7 uses a transformable sheet material for a a stimulable phosphor layer 114. The transparent moisture-resistant protective layer 118 has plural pockets 152 that serve as buffer spaces.
According to the stimulable phosphor panel in US 2003/016188 A, changes in volume of the airtight area where the stimulable phosphor layer is formed due to changes in temperature and atmospheric pressure can be absorbed by, in the stimulable phosphor panel 140 of FIG. 6, moving the piston 142 and changing the volume of the buffer space 148 or, in the stimulable phosphor panel 150 of FIG. 7 making the pockets 152 expand or contract. Thus, the stimulable phosphor panels in US 2003/0160188 A can advantageously prevent a change in volume of the airtight space from causing the aforementioned problems including a change in distance from the transparent moisture-resistant protective layer 118 to the stimulable phosphor layer 114 and warping of the stimulable phosphor layer 114.
However, every stimulable phosphor panel disclosed in US 2003/0160188 A has a buffer space located outside the panel. Depending on the size and use of the stimulable phosphor panel, and environment in which the stimulable phosphor panel is used, enough buffer space may not be secured, or the stimulable phosphor panel has to be often uspsized to secure enough buffer space.